Optical interrogator for performing interferometry using Bragg gratings

ABSTRACT

An optical fiber interrogator for interrogating optical fiber that includes fiber Bragg gratings (“FBGs”). The interrogator includes a light source operable to emit phase coherent light, amplitude modulation circuitry optically coupled to the light source and operable to generate pulses from the light, and control circuitry communicatively coupled to the amplitude modulation circuitry that is configured to perform a method for interrogating the optical fiber. The method includes generating a pair of light pulses by using the amplitude modulation circuitry to modulate light output by the light source without splitting the light.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/532,963, filed Jun. 2, 2017, which is the U.S. National Stage ofInternational Application No. PCT/CA2015/051269, filed Dec. 4, 2015,which claims the benefit of U.S. Provisional Application No. 62/207,251,filed Aug. 19, 2015, and U.S. Provisional Application No. 62/087,669,filed Dec. 4, 2014. The prior applications are incorporated herein byreference in their entirety.

FIELD

The present disclosure is directed at an optical interrogator forperforming interferometry using fiber Bragg gratings.

BACKGROUND

Optical interferometry is a technique in which two separate lightpulses, a sensing pulse and a reference pulse, are generated andinterfere with each other. When optical interferometry is used for fiberoptic sensing applications, the sensing and reference pulses are atleast partially reflected back towards an optical receiver. For example,optical interferometry may be performed by directing the sensing andreference pulses along an optical fiber that comprises fiber Bragggratings, which partially reflect the pulses back towards an opticalreceiver at which interference is observed. The nature of theinterference observed at the optical receiver provides information onthe optical path length the pulses traveled, which in turn providesinformation on parameters such as the strain the optical fiberexperienced.

The circuitry that generates, modulates, and receives the sensing andreference pulses is typically contained within a device called anoptical interrogator. There exists a continued desire to advance andimprove technology used in optical interrogators.

SUMMARY

According to a first aspect, there is provided an optical fiberinterrogator for interrogating optical fiber comprising fiber Bragggratings (“FBGs”). The interrogator comprises a light source operable toemit phase coherent light; amplitude modulation circuitry opticallycoupled to the light source and operable to generate pulses from thelight, wherein the pulses are generated without splitting the light; andcontrol circuitry comprising a controller, communicatively coupled tothe amplitude modulation circuitry, configured to perform a method forinterrogating the optical fiber comprising generating a pair of lightpulses by using the amplitude modulation circuitry to modulate lightoutput by the light source.

The interrogator may further comprise a phase modulator opticallycoupled to the amplitude modulation circuitry and operable to introducea phase shift to at least one of the pulses, and the method may furthercomprise phase shifting at least one of the light pulses relative to theother of the light pulses by using the phase modulator.

The phase modulator may be selected from the group consisting of alithium niobate phase modulator, a gallium arsenide phase modulator, andan indium phosphide phase modulator.

The interrogator may further comprise an output optical amplifieroptically coupled to the phase modulator; receiver circuitry; and anoptical circulator comprising first, second, and third ports, whereinthe first port is optically coupled to the output optical amplifier, asecond port is optically coupled to an output of the interrogator forrespectively sending and receiving the pulses to and from the opticalfiber, and a third port is optically coupled to the receiver circuitryfor processing signals received from the optical fiber.

The interrogator may further comprise polarization maintaining fiberbetween the light source and the output such that the polarization ofthe light is maintained from the light source to the output.

The interrogator may further comprise polarization maintaining fiberbetween the output and the receiver circuitry such that the polarizationof reflections off the FBGs are maintained from the output to thereceiver circuitry.

The interrogator may further comprise a polarization controlleroptically coupled between the phase modulator and the output opticalamplifier.

The interrogator may further comprise a polarization splitter opticallycoupled between the third port of the optical circulator and thereceiver circuitry.

The interrogator may further comprise receiver circuitry; and an opticalcirculator comprising first, second, and third ports, wherein the firstport is optically coupled to the phase modulator, a second port isoptically coupled to an output of the interrogator for respectivelysending and receiving the pulses to and from optical fiber, and a thirdport is optically coupled to the receiver circuitry for processingsignals received from the optical fiber.

The light source may comprise a laser having a power of at least 100 mW.

The phase shifting may comprise applying a positive phase shift to afirst pulse and applying a negative phase shift to a subsequent, secondpulse intended to interfere with the first pulse.

The first and second pulses may differ in phase from each other by morethan π radians.

The method may further comprise generating a calibration pulse;determining when reflections of the calibration pulse off the FBGsarrive at the receiver circuitry; and based on differences in when thereflections of the calibration pulse arrive at the receiver circuitry,determining timing between the sensing and reference pulses.

The phase shifting may comprise applying a non-linear phase shift or apiecewise linear phase shift to at least one of the pulses.

The phase shift may be a Barker code.

The method may further comprise dithering leakage from the amplitudemodulation circuitry by phase shifting the leakage between 0 and πradians at a frequency at least 2.5 times higher than a frequency atwhich interrogation is being performed.

The amplitude modulation circuitry may comprise an input opticalisolator and an output optical isolator isolating an input and output ofthe amplitude modulation circuitry, respectively; an optical attenuatoroptically coupled between the input and output isolators; and a firstoptical amplifier optically coupled between the attenuator and theoutput isolator.

The light source may comprise an electroabsorption modulated laser andthe amplitude modulation circuitry may comprise an absorption region ofthe electroabsorption modulated laser.

According to another aspect, there is provided a system forinterrogating optical fiber comprising fiber Bragg gratings (“FBGs”)comprising any foregoing aspect of the interrogator optically coupled tothe optical fiber, which is polarization maintaining fiber.

According to another aspect, there is provided a method forinterrogating optical fiber comprising fiber Bragg gratings (“FBGs”).The method comprises generating a pair of light pulses from phasecoherent light emitted from a light source, wherein the light pulses aregenerated by modulating the intensity of the light without splitting thelight; transmitting the light pulses along the optical fiber; receivingreflections of the pulses off the FBGs; and determining whether anoptical path length between the FBGs has changed from an interferencepattern resulting from the reflections of the pulses.

Determining whether the optical path length has changed may compriseconverting the interference pattern from an optical to an electricalsignal.

The method may further comprise phase shifting at least one of the lightpulses relative to the other of the light pulses.

A phase modulator may be used to phase shift at least one of the lightpulses, and the phase modulator may be selected from the groupconsisting of a lithium niobate phase modulator, a gallium arsenidephase modulator, and an indium phosphide phase modulator.

Polarization of the light pulses may be maintained from when the lightpulses are generated until the light pulses are transmitted along theoptical fiber.

Polarization of the light pulses may be maintained from when the lightpulses are generated until the interference pattern resulting from thereflections of the pulses is observed.

The method may further comprise splitting the polarization of thereflected pulses prior to converting the interference patterns.

The light source may be a laser and the intensity of the light may bemodulated using a first optical amplifier external of and opticallycoupled to the laser.

The light may be generated by an electroabsorption modulated laser andthe intensity of the light may be modulated using an absorption regioncomprising part of the laser.

The light source may comprise a laser having a power of at least 100 mW.

The phase shifting may comprise applying a positive phase shift to afirst pulse and applying a negative phase shift to a subsequent, secondpulse intended to interfere with the first pulse.

The first and second pulses may differ in phase from each other by morethan π radians.

The method may further comprise transmitting a calibration pulse to theFBGs; receiving reflections of the calibration pulse off the FBGs; andbased on differences in when the reflections of the calibration pulseare received, determining timing between the sensing and referencepulses.

The phase shifting may comprise applying a non-linear phase shift or apiecewise linear phase shift to at least one of the pulses.

The phase shift may be a Barker code.

The method may further comprise dithering leakage from the light sourceby phase shifting the leakage between 0 and π radians at a frequency atleast 2.5 times higher than a frequency at which interrogation is beingperformed.

According to another aspect, there is provided a non-transitory computerreadable medium having stored thereon program code to cause a processorto perform a method according to any of the above aspects or suitablecombinations thereof for interrogating optical fiber comprising fiberBragg gratings (“FBGs”).

This summary does not necessarily describe the entire scope of allaspects. Other aspects, features and advantages will be apparent tothose of ordinary skill in the art upon review of the followingdescription of specific embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, which illustrate one or more exampleembodiments:

FIG. 1A is a block diagram of a system for detecting dynamic strain,which includes an optical fiber with fiber Bragg gratings (“FBGs”) forreflecting a light pulse, according to one embodiment.

FIG. 1B is a schematic that depicts how the FBGs reflect a light pulse.

FIG. 1C is a schematic that depicts how a light pulse interacts withimpurities in an optical fiber that results in scattered laser light dueto Rayleigh scattering, which is used for distributed acoustic sensing(“DAS”).

FIG. 2 is a schematic of an optical interrogator for performinginterferometry using FBGs, according to the prior art.

FIGS. 3 to 5 and 9A are schematics of an optical interrogator forperforming interferometry using FBGs, according to various embodiments.

FIG. 6 is a graph of example pulses resulting from reflections ofsensing and reference pulses off of the FBGs.

FIG. 7 is a method for interrogating optical fiber that comprises FBGs,according to another embodiment.

FIG. 8 is a method for calibrating the optical interrogator, accordingto another embodiment.

FIG. 9B depicts an example of pulse timing applicable to the opticalinterrogator of FIG. 9A.

DETAILED DESCRIPTION

Directional terms such as “top”, “bottom”, “upwards”, “downwards”,“vertically”, and “laterally” are used in the following description forthe purpose of providing relative reference only, and are not intendedto suggest any limitations on how any article is to be positioned duringuse, or to be mounted in an assembly or relative to an environment.Additionally, the term “couple” and variants of it such as “coupled”,“couples”, and “coupling” as used in this description are intended toinclude indirect and direct connections unless otherwise indicated. Forexample, if a first device is coupled to a second device, that couplingmay be through a direct connection or through an indirect connection viaother devices and connections. Similarly, if the first device iscommunicatively coupled to the second device, communication may bethrough a direct connection or through an indirect connection via otherdevices and connections.

Optical interferometry is a technique in which two separate light pulsesare generated: a sensing pulse and a reference pulse. These pulses maybe generated by an optical source such as a laser. When opticalinterferometry is used for fiber optic sensing applications, the sensingand reference pulses are at least partially reflected back towards anoptical receiver. Optical interferometry has a variety of applications,one of which is being used to detect dynamic strain.

Referring now to FIG. 1A, there is shown one embodiment of a system 100for performing interferometry using fiber Bragg gratings (“FBGs”). Thesystem 100 comprises optical fiber 112, an interrogator 106 opticallycoupled to the optical fiber 112, and a signal processing device 118that is communicative with the interrogator 106.

The optical fiber 112 comprises one or more fiber optic strands, each ofwhich is made from quartz glass (amorphous SiO₂). The fiber opticstrands are doped with various elements and compounds (includinggermanium, erbium oxides, and others) to alter their refractive indices,although in alternative embodiments the fiber optic strands may not bedoped. Single mode and multimode optical strands of fiber arecommercially available from, for example, Corning® Optical Fiber.Example optical fibers include ClearCurve™ fibers (bend insensitive),SMF28 series single mode fibers such as SMF-28 ULL fibers or SMF-28efibers, and InfiniCor® series multimode fibers.

The interrogator 106 generates the sensing and reference pulses andoutputs the reference pulse after the sensing pulse. The pulses aretransmitted along optical fiber 112 that comprises a first pair of FBGs.The first pair of FBGs comprises first and second FBGs 114 a,b(generally, “FBGs 114”). The first and second FBGs 114 a,b are separatedby a certain segment 116 of the optical fiber 112 (“fiber segment 116”).The optical length of the fiber segment 116 varies in response todynamic strain that the fiber segment 116 experiences.

The light pulses have a wavelength identical or very close to the centerwavelength of the FBGs 114, which is the wavelength of light the FBGs114 are designed to partially reflect; for example, typical FBGs 114 aretuned to reflect light in the 1,000 to 2,000 nm wavelength range. Thesensing and reference pulses are accordingly each partially reflected bythe FBGs 114 a,b and return to the interrogator 106. The delay betweentransmission of the sensing and reference pulses is such that thereference pulse that reflects off the first FBG 114 a (hereinafter the“reflected reference pulse”) arrives at the optical receiver 103simultaneously with the sensing pulse that reflects off the second FBG114 b (hereinafter the “reflected sensing pulse”), which permits opticalinterference to occur.

While FIG. 1A shows only the one pair of FBGs 114 a,b, in alternativeembodiments (not depicted) any number of FBGs 114 may be on the fiber112, and time division multiplexing (TDM) (and optionally, wavelengthdivision multiplexing (WDM)) may be used to simultaneously obtainmeasurements from them. If two or more pairs of FBGs 114 are used, anyone of the pairs may be tuned to reflect a different center wavelengththan any other of the pairs. Alternatively a group of multiple FBGs 114may be tuned to reflect a different center wavelength to another groupof multiple FBGs 114 and there may be any number of groups of multipleFBGs extending along the optical fiber 112 with each group of FBGs 114tuned to reflect a different center wavelength. In these exampleembodiments where different pairs or group of FBGs 114 are tuned toreflect different center wavelengths to other pairs or groups of FBGs114, WDM may be used in order to transmit and to receive light from thedifferent pairs or groups of FBGs 114, effectively extending the numberof FBG pairs or groups that can be used in series along the opticalfiber 112 by reducing the effect of optical loss that otherwise wouldhave resulted from light reflecting from the FBGs 114 located on thefiber 112 nearer to the interrogator 106. When different pairs of theFBGs 114 are not tuned to different center wavelengths, TDM issufficient.

The interrogator 106 emits laser light with a wavelength selected to beidentical or sufficiently near the center wavelength of the FBGs 114that each of the FBGs 114 partially reflects the light back towards theinterrogator 106. The timing of the successively transmitted lightpulses is such that the light pulses reflected by the first and secondFBGs 114 a,b interfere with each other at the interrogator 106, whichrecords the resulting interference signal. The strain that the fibersegment 116 experiences alters the optical path length between the twoFBGs 114 and thus causes a phase difference to arise between the twointerfering pulses. The resultant optical power at the optical receiver103 can be used to determine this phase difference. Consequently, theinterference signal that the interrogator 106 receives varies with thestrain the fiber segment 116 is experiencing, which allows theinterrogator 106 to estimate the strain the fiber segment 116experiences from the received optical power. The interrogator 106digitizes the phase difference (“output signal”) whose magnitude andfrequency vary directly with the magnitude and frequency of the dynamicstrain the fiber segment 116 experiences.

The signal processing device 118 is communicatively coupled to theinterrogator 106 to receive the output signal. The signal processingdevice 118 includes a processor 102 and a non-transitory computerreadable medium 104 that are communicatively coupled to each other. Aninput device 110 and a display 108 interact with the processor 102. Thecomputer readable medium 104 has stored on it program code to cause theprocessor 102 to perform any suitable signal processing methods to theoutput signal. For example, if the fiber segment 116 is laid adjacent aregion of interest that is simultaneously experiencing vibration at arate under 20 Hz and acoustics at a rate over 20 Hz, the fiber segment116 will experience similar strain and the output signal will comprise asuperposition of signals representative of that vibration and thoseacoustics. The processor 102 may apply a low pass filter with a cutofffrequency of 20 Hz to the output signal to isolate the vibration portionof the output signal from the acoustics portion of the output signal.Analogously, to isolate the acoustics portion of the output signal fromthe vibration portion, the processor 102 may apply a high pass filterwith a cutoff frequency of 20 Hz. The processor 102 may also apply morecomplex signal processing methods to the output signal; example methodsinclude those described in PCT application PCT/CA2012/000018(publication number WO 2013/102252), the entirety of which is herebyincorporated by reference.

FIG. 1B depicts how the FBGs 114 reflect the light pulse, according toanother embodiment in which the optical fiber 112 comprises a third FBG114 c. In FIG. 1B, the second FBG 114 b is equidistant from each of thefirst and third FBGs 114 a,c when the fiber 112 is not strained. Thelight pulse is propagating along the fiber 112 and encounters threedifferent FBGs 114, with each of the FBGs 114 reflecting a portion 115of the pulse back towards the interrogator 106. In embodimentscomprising three or more FBGs 114, the portions of the sensing andreference pulses not reflected by the first and second FBGs 114 a,b canreflect off the third FBG 114 c and any subsequent FBGs 114, resultingin interferometry that can be used to detect strain along the fiber 112occurring further from the interrogator 106 than the second FBG 114 b.For example, in the embodiment of FIG. 1B, a portion of the sensingpulse not reflected by the first and second FBGs 114 a,b can reflect offthe third FBG 114 c and a portion of the reference pulse not reflectedby the first FBG 114 a can reflect off the second FBG 114 b, and thesereflected pulses can interfere with each other at the interrogator 106.

Any changes to the optical path length of the fiber segment 116 resultin a corresponding phase difference between the reflected reference andsensing pulses at the interrogator 106. Since the two reflected pulsesare received as one combined interference pulse, the phase differencebetween them is embedded in the combined signal. This phase informationcan be extracted using proper signal processing techniques, such asphase demodulation. The relationship between the optical path of thefiber segment 116 and that phase difference (θ) is as follows:

$\theta = \frac{2\;\pi\;{nL}}{\lambda}$where n is the index of refraction of the optical fiber; L is thephysical path length of the fiber segment 116; and λ is the wavelengthof the optical pulses. A change in nL is caused by the fiberexperiencing longitudinal strain induced by energy being transferredinto the fiber. The source of this energy may be, for example, an objectoutside of the fiber experiencing dynamic strain, undergoing vibration,or emitting energy. As used herein, “dynamic strain”, refers to strainthat changes over time. Dynamic strain that has a frequency of betweenabout 5 Hz and about 20 Hz is referred to by persons skilled in the artas “vibration”, dynamic strain that has a frequency of greater thanabout 20 Hz is referred to by persons skilled in the art as “acoustics”,and dynamic strain that changes at a rate of <1 Hz, such as at 500 μHz,is referred to as “sub-Hz strain”.

One conventional way of determining Δ nL is by using what is broadlyreferred to as distributed acoustic sensing (“DAS”). DAS involves layingthe fiber 112 through or near a region of interest and then sending acoherent laser pulse along the fiber 112. As shown in FIG. 1C, the laserpulse interacts with impurities 113 in the fiber 112, which results inscattered laser light 117 because of Rayleigh scattering. Vibration oracoustics emanating from the region of interest results in a certainlength of the fiber becoming strained, and the optical path change alongthat length varies directly with the magnitude of that strain. Some ofthe scattered laser light 117 is back scattered along the fiber 112 andis directed towards the optical receiver 103, and depending on theamount of time required for the scattered light 117 to reach thereceiver and the phase of the scattered light 117 as determined at thereceiver, the location and magnitude of the vibration or acoustics canbe estimated with respect to time. DAS relies on interferometry usingthe reflected light to estimate the strain the fiber experiences. Theamount of light that is reflected is relatively low because it is asubset of the scattered light 117. Consequently, and as evidenced bycomparing FIGS. 1B and 1C, Rayleigh scattering transmits less light backtowards the optical receiver 103 than using the FBGs 114.

DAS accordingly uses Rayleigh scattering to estimate the magnitude, withrespect to time, of the strain experienced by the fiber during aninterrogation time window, which is a proxy for the magnitude of thevibration or acoustics emanating from the region of interest. Incontrast, the embodiments described herein measure dynamic strain usinginterferometry resulting from laser light reflected by FBGs 114 that areadded to the fiber 112 and that are designed to reflect significantlymore of the light than is reflected as a result of Rayleigh scattering.This contrasts with an alternative use of FBGs 114 in which the centerwavelengths of the FBGs 114 are monitored to detect any changes that mayresult to it in response to strain. In the depicted embodiments, groupsof the FBGs 114 are located along the fiber 112. A typical FBG can havea reflectivity rating of 2% or 5%. The use of FBG-based interferometryto measure dynamic strain offers several advantages over DAS, in termsof optical performance.

FIG. 2 is a schematic of an example prior art interrogator 10 that maybe used to perform FBG-based interferometry. The interrogator 10comprises a narrowband light source 12 optically coupled via a singleoptical path 32 to a first optical coupler 14. The first optical coupler14 splits any pulses emitted from the light source 12 into the sensingpulse, directed along a lower optical path 30, and the reference pulse,directed along an upper optical path 28. The upper optical path 28comprises a loop of coiled fiber 16 that delays the reference pulserelative to the sensing pulse based on the spacing of the FBGs 114. Thelower optical path 30 comprises a piezoelectric fiber stretcher 18,which is used to phase modulate the sensing pulse. The upper and loweroptical paths 28,30 are collectively referred to as the interrogator's10 “compensator”. At the end of the compensator is a second opticalcoupler 20 that directs both pulses back along the single optical path32. Between the second optical coupler 20 and the output of theinterrogator 10 are an optical amplifier 22 and an optical circulator24. Before leaving the interrogator 10, the sensing and reference pulsesare amplified by the optical amplifier 22 and pass through the opticalcirculator 24. They are then transmitted to and reflect off of the FBGs114 as described above in respect of FIGS. 1A-1C and return to theinterrogator 10. Upon encountering the optical circulator 24 thereflected pulses are directed to receiver circuitry 26 and to the signalprocessing device 118 where any interference pattern can be analyzed.

This prior art interrogator 10 suffers from a variety of problems, suchas the following:

-   -   1. signal-to-noise ratio (“SNR”) is prejudiced by splitting the        light pulse emitted by the light source 12 at the first optical        coupler 14 in order to create the sensing and reference pulses;        and    -   2. by virtue at least in part of modulating through mechanical        movement, the piezoelectric fiber stretcher 18:        -   a. is relatively slow and is in practice used only to            sinusoidally and approximately linearly (by using a            relatively small subset of a sinusoidal modulation profile)            modulate the sensing pulse; and        -   b. introduces significant noise (mechanical noise and            electrical noise resulting from high voltage power supplies            used to power the stretcher 18), vibration, signal jitter,            and birefringence to signal measurement, further prejudicing            SNR.

The embodiments described herein are directed at improving upon at leastone of the problems experienced by the prior art interrogator 10. Moreparticularly, the embodiments described herein are directed at aninterrogator in which generating the sensing and reference pulses isdone without splitting a light pulse, which helps achieve a relativelyhigh SNR. The embodiments described herein also do not use thepiezoelectric fiber stretcher 18 to modulate the phase of the sensingpulse; instead, some of the embodiments use a solid state phasemodulator, such as a lithium niobate phase modulator, that permits thesensing pulse to be non-linearly modulated and that introduces lessnoise and allows a more accurate phase determination than thepiezoelectric fiber stretcher 18.

Referring now to FIG. 3, there is shown an optical interrogator 300 forperforming interferometry using FBGs, according to one embodiment. Theinterrogator 300 comprises a light source in the form of a laser 302whose output is optically coupled in series to various opticalcomponents; in order from the laser 302 these components are an inputoptical isolator 304 a, an optical attenuator 306, a first opticalamplifier 308, an output optical isolator 304 b, a phase modulator 310,an output optical amplifier 314, and a first port of an opticalcirculator 320. A second port of the optical circulator 320 is opticallycoupled to the interrogator's 300 output. Optically coupled to theinterrogator's 300 output is the optical fiber 112 comprising the FBGs114. A third port of the optical circulator 320 is optically coupled toreceiver circuitry 322, which in the depicted embodiment convertsreflected light pulses into electrical signals but which in alternativeembodiments may convert the reflected light pulses into a different typeof signal, such as an acoustic signal. The optical circulator 320directs light pulses entering its first port out its second port, anddirects light pulses entering its second port out its third port. Theeffect of this is that the sensing and reference pulses are transmittedfrom the output optical amplifier 314 to the FBGs 114, while reflectedpulses are transmitted from the FBGs 114 to the receiver circuitry 322.The optical fiber 112 is used to optically couple the components thatcomprise the laser 302, optical isolators 304 a,b, optical attenuator306, optical amplifiers 308,314, phase modulator 310, optical circulator320, and receiver circuitry 322 together. However, in an alternativeembodiment (not depicted) an alternative to the optical fiber 112 may beused to optically couple the various components together; for example,free space optical communication may be used to optically couple thevarious components together. In another alternative embodiment (notdepicted), the optical circulator 320 may be replaced with a packagecomprising an optical coupler and an optical isolator.

In FIG. 3 the laser 302 outputs phase coherent light to permit thesensing and reflected pulses to interfere with each other after beingreflected by the FBGs 114. More particularly, in one embodiment thelaser 302 outputs phase coherent light during transmission of thesensing and reference pulses so that at least the sensing and referencepulses are phase coherent with each other; that is, the laser's 302coherence time is at least as long as the time required to generate apair of sensing and reference pulses. In an alternative embodiment, thelaser 302 may have a longer coherence time; for example, the laser 302may produce coherent light for at least the entire duration ofinterrogation (i.e., the time between generation of the first pulse andthe last recorded interference pattern between pulses); for at least acertain multiple (e.g. ten times) of the duration that the sensing andreference pulses are generated for transmission along the optical fiber112; or the laser 302 may always generate coherent light whenever inoperation. Additionally, while the laser 302 is the light source in thedepicted embodiment, alternative embodiments (not depicted) may comprisea non-laser coherent light source.

The interrogator 300 also comprises a controller 324 communicativelycoupled to the first optical amplifier 308 and to the phase modulator310 via a digital to analog converter 326 (“DAC 326”) and an analogamplifier 328. The controller 324 is consequently able to control theamplitude and phase modulation of the sensing and reference pulses. Thecontroller 324 is configured to perform a method for interrogating theFBGs 114 or for calibrating the interrogator 300, such as the examplemethods shown in FIGS. 7 and 8 and described in more detail, below. Thecontroller 324 in the depicted embodiment is a field programmable gatearray (“FPGA”), which is configured using a hardware descriptionlanguage such as VHDL or Verilog from which a netlist is generated andused to configure the FPGA in the field. The DAC 326 and analogamplifier 328 allow the controller 324 to output all digital signals andstill be able to control the first optical amplifier 308 and phasemodulator 310; in an alternative embodiment (not depicted) some or allof the signals the controller 324 outputs may be analog signals and thecontroller 324 may consequently be directly communicatively coupled toone or both of the amplifier 308 and phase modulator 310. Alternatively,one or both of the amplifier 308 and phase modulator 310 may beconfigured to receive digital input signals, in which case thecontroller 324 may be directly communicatively coupled to one or both ofthe amplifier 308 and phase modulator 310 if the controller 324 alsooutputs at least some digital signals. As another alternative (notdepicted), one or both of the amplifier 308 and the phase modulator 310may be configured to receive analog signals, the controller 324 may beconfigured to output at least some analog signals, and the controller324 may be communicatively coupled to one or both of the amplifier 308and phase modulator via an analog to digital converter and, optionally,a digital amplifier.

In this depicted embodiment, the laser 302 generates light centered on1,550 nm and has a narrow line width and a long coherence length. Theinput optical isolator 304 a prevents back reflections fromdestabilizing the laser 302. The optical attenuator 306 allows theintensity of the laser light to be varied so as not to saturate thefirst optical amplifier 308, which in this example embodiment is asemiconductor optical amplifier (“SOA”). The output optical isolator 304b prevents back reflections from destabilizing the first opticalamplifier 308. The phase modulator 310, which in this example embodimentis a solid state lithium niobate phase modulator, allows the controller324 to control phase modulation of one or both of the sensing andreference pulses. The output optical amplifier 314 boosts the power ofthe sensing and reference pulses for transmission to the FBGs 114; inthis example embodiment, the output optical amplifier 314 is an erbiumdoped fiber amplifier (“EDFA”).

Example component manufacturers are Covega™ Technologies for the firstoptical amplifier 308 and the phase modulator 310, Nuphoton™Technologies, Inc. for the output optical amplifier 314, OSI™ LaserDiode Inc. for the receiver circuitry 322, OZ Optics™ Ltd. for thecirculator 320, and Thorlabs™, Inc. for the optical isolators 304 a,b.

Referring now to FIG. 7, there is shown a method 700 for interrogatingthe optical fiber 112, according to another embodiment. As mentionedabove, the method 700 is encoded on to the FPGA that comprises thecontroller 324 as a combination of FPGA elements such as logic blocks.The controller 324 begins performing the method 700 at block 702 andproceeds to block 704 where it generates a pair of light pulses usinglight emitted from a light source by modulating the intensity of thelight without splitting the light; in the interrogator 300 of FIG. 3,these light pulses are the sensing and reference pulses and the lightsource is the laser 302. To generate the sensing and reference pulsesthe controller 324 controls the first optical amplifier 308 to modulatethe amplitude of the light the laser 302 emits. Modulating the lightwithout splitting the light as done in the prior art interrogator 10facilitates the interrogator 300 in FIG. 3 having a higher SNR than theprior art interrogator 10 because generating the sensing and referencepulses does not comprise halving the input intensity of light bysplitting a light pulse along the upper and lower paths 28,30. Theamplitude modulation used to generate the pair of light pulses withoutsplitting the light may comprise, for example, one or both of absorbingand reflecting the light.

After being generated, the pulses are amplified by the output opticalamplifier 314 and are transmitted through the optical circulator 320 andto the optical fiber 112 and the FBGs 114 (block 706). The pulses arethen reflected off the FBGs 114 and return to the interrogator 300(block 708) where they are directed via the optical circulator 320 tothe receiver circuitry 322, which in the depicted embodiment convertsthe interference pattern resulting from the reflections into anelectrical signal. The interference patterns resulting from thereflections are then observed, such as at the signal processing software118, and an operator of the interrogator 300 can determine whether theoptical path length between the FBGs 114 has changed from theinterference pattern that results from interference of the reflections(block 710). For example, the operator can make determinations about thenature of the dynamic strain experienced by the fiber segments 116between the FBGs 114.

In some alternative embodiments, between blocks 704 and 706 thecontroller 324 phase shifts one of the light pulses relative to theother of the light pulses; that is, in the example embodiment in whichthe sensing and reference pulses are generated, the controller 324causes the phase modulator 310 to phase shift one or both of the sensingand reference pulses. When the phase modulator 310 is a lithium niobatephase modulator, the modulator 310 is able to introduce a phase shift ofup to +/−π to one or both of the sensing and reference pulses; byintroducing a phase shift of as much as +π to one of the pulses and asmuch as −π to the other of the pulses, the controller 324 can introducea phase difference of anywhere from 0 to 2π between the pulses. Incontrast to the conventional piezoelectric fiber stretcher 18, using alithium niobate phase modulator permits faster phase modulation rates(in the depicted embodiment, the phase modulator 308 can modulate at upto 10 GHz, and alternative and commercially available phase modulators308 can modulate at up to 40 GHz), introduces less noise, and permitsnon-linear modulation schemes. A lithium niobate phase modulator permitsnon-linear and piecewise linear modulation schemes; for example, any ofa sinusoidal, sawtooth, triangle, and stepwise function can be used todrive the phase modulator 310, with the light pulses being modulatedaccordingly. In another alternative embodiment, a Barker code may beused for phase modulation.

However, even without phase shifting one or both of the pulses theinterrogator 300 is able to interrogate the optical fiber 112. Byindependently generating two light pulses without splitting a singlepulse, the interrogator 300 is able to generate pulses of approximatelytwice the power than if two pulses were generated by splitting a singlepulse as is done in the prior art interrogator 10. Additionally,generating two pulses using the amplitude modulation circuitry of theinterrogator 300 allows finer timing control, regardless of phasemodulation, than the prior art interrogator 10 and also permits thesensing and reference pulses to be generated with a variety of differentamplitudes, including amplitudes that are different from each other. Inthe prior art interrogator 10, the sensing and reference pulsestypically have identical amplitudes because they are generated bysplitting a pulse from the light source 12 in half.

The embodiments of the interrogator 300 shown in FIGS. 3-5 may be usedwithout activating the phase modulator 310 to phase shift the sensing orreference pulses relative to each other, as described above in respectof FIG. 7. In alternative embodiments (not depicted), the interrogator300 may be constructed without the phase modulator 310 and accordinglybe designed for amplitude modulation only. For example, alternativeembodiments of the interrogator 300 may be based on or identical to theembodiments of FIGS. 3-5 except that they may be missing the phasemodulator 310.

As alluded to above in respect of FIG. 1A, in some alternativeembodiments (not depicted) the fiber 112 may comprise groups of two ormore of the FBGs 114, with these groups located at different positionsalong the fiber 112 and with the FBGs 114 in any one of these groupstuned to a common center wavelength that is different from the centerwavelength to which the FBGs 114 in the other groups are tuned. Forexample, there may be a first group of three FBGs 114 along the fiber112 extending from 200 m to 250 m from the interrogator 300 and tuned toa first center wavelength, a second group of three FBGs 114 along thefiber 112 extending from 400 m to 450 m from the interrogator 300 andtuned to a second center wavelength different from the first centerwavelength, and a third group of three FBGs 114 along the fiber 112extending from 600 m to 650 m from the interrogator 300 and tuned to athird center wavelength different from the first and second centerwavelengths. In this example, the controller 324 may be configured tocause the interrogator 300 to use TDM to interrogate each of these threedifferent groups of FBGs 114 using pulses of the three differentwavelengths of light launched from the interrogator 300 at differenttimes. For example, a first pair of sensing and reference pulses at thefirst center wavelength may be launched for the first group of FBGs 114at times t₁ and t₂, a second pair of sensing and reference pulses at thesecond center wavelength may be launched for the second group of FBGs114 at times t₃ and t₄, and a third pair of sensing and reference pulsesat the third center wavelength may be launched for the third group ofFBGs 114 at times t₅ and t₆, with t₁<t₂<t₃<t₄<t₅<t₆. In this mannerdifferent wavelengths of light may be used to interrogate differentlengths of the fiber 112. In an alternative embodiment, light pulseshaving different wavelengths may be simultaneously launched into thefiber 112; in this embodiment and applying the terminology of theimmediately preceding example, t₁=t₃=t₅ and t₂=t₄=t₆, with each of t₁,t₃, and t₅>t₂, t₄, and t₆.

Example interference patterns are depicted in FIG. 6. FIG. 6 shows agraph 600 of first through fourth pulses 602 a-d (collectively, “pulses602”) resulting from reflections off of the FBGs 114 of the sensing andreference pulses generated using the interrogator 300 of FIG. 3. Thepulses 602 are measured after the receiver circuitry 322 has convertedthe reflections from an optical to an electrical signal.

The graph 600 is generated by interrogating three of the FBGs 114: thefirst and second FBGs 114 a,b and a third FBG 114 (not depicted in FIG.3) located along the optical fiber 112 further from the interrogator 300than the second FBG 114 b, with the three FBGs 114 equally spaced fromeach other. The first pulse 602 a shows the sensing pulse after it hasreflected off of the first FBG 114 a; the second pulse 602 b shows theinterference resulting from the reference pulse after it has reflectedoff the first FBG 114 a and the sensing pulse after it has reflected offthe second FBG 114 b; the third pulse 602 c shows the interferenceresulting from the reference pulse after it has reflected off the secondFBG 114 b and the sensing pulse after it has reflected off the third FBG114 c; and the fourth pulse 602 d shows the reference pulse after it hasreflected off the third FBG 114.

Any variation in the optical length of the fiber segment 116 between thefirst and second FBGs 114 a,b is reflected in the phase variation of thesecond pulse 602 b. Similarly, any variation in the optical length ofthe fiber segment 116 between the second FBG 114 b and the third FBG 114is reflected in the amplitude variation of the third pulse 602 c. Asdiscussed above in respect of FIGS. 1A-1C, the optical length of thefiber 112 can be changed in response to dynamic strain, of which onetype is strain in the fiber 112 caused by an acoustic signal.

Alternative Embodiments

In addition to the example embodiment of the interrogator 300 shown inFIG. 3, alternative embodiments are possible. Example alternativeembodiments of the interrogator 300 are shown in FIGS. 4 and 5.

FIG. 4 shows an embodiment of the interrogator 300 in which apolarization controller 404 is optically coupled between the phasemodulator 310 and the output optical amplifier 314 and in which apolarization splitter 402 is optically coupled between the opticalcirculator 320 and the receiver circuitry 322. In FIG. 4, the outputoptical amplifier 314 and the optical circulator 320 are polarizationmaintaining components, and all the fiber 112 between the polarizationcontroller 404 and the FBGs 114 (including the fiber segment 116) andbetween the polarization controller 404 and the polarization splitter402 is polarization maintaining fiber (“PMF”). An example brand of PMFis Panda Fiber™ manufactured by Fujikura™ Ltd. The polarizationcontroller 404 is actively controlled by, and accordinglycommunicatively coupled to, the controller 324. Regardless of thepolarization of the light entering the polarization controller 404, thepolarization controller 404 converts the polarization of any laser lightexiting the phase modulator 310 into a known polarization, which the PMFmaintains. Both the sensing and reference pulses will consequently enterthe output optical amplifier 314 in the same polarization state, and anychanges in polarization between the output optical amplifier 314 and thereceiver circuitry 322 will be experienced by both pulses except for anypolarization changes occurring in the fiber segments 116 between pairsof the FBGs 114. This helps to keep the polarizations of the sensing andreference pulses aligned, which increases the degree to which the pulsesinterfere and consequently the sensitivity of the interrogator 300. Thepolarization splitter 402 allows either all reflected light or any oneof three polarizations of reflected light, each separated by 120°, topass through to the receiver circuitry 322 while discarding theremaining polarizations. Permitting only one polarization to reach thereceiver circuitry 322 allows the receiver circuitry 322 to discardnoisy data that could reduce the interrogator's 300 sensitivity andaccuracy. The polarization splitter 402 can also be used to permit anycombination of the three polarizations of the reflected light, such asthe sum of any two or all three polarizations of the reflected light, toreach the receiver circuitry 322 if desired.

The polarization controller 404 in FIG. 4 increases component selectionflexibility by permitting selection of a wider range of lasers than whenthe polarization controller 404 is not used. Commercially availablelasers may or may not output light of a fixed polarization; thepolarization controller 404 allows polarization of the laser 302 to beadjusted. Accordingly, the laser 302 need not emit light of a constantand known polarization in order for the interrogator 302 to emit lightof a known polarization to the FBGs 114. Similarly, the polarizationcontroller 404 allows non-PMF to be used between the laser 302 and thepolarization controller 404 and allows the optical components betweenthe laser 302 and the polarization controller 404 to not be polarizationmaintaining while still permitting the interrogator 300 to enjoy atleast some benefits of polarization control. In an alternativeembodiment (not depicted), the polarization controller 404 can beomitted from the interrogator 300 of FIG. 4 and the laser 302 can beconfigured to output a known and fixed polarization and be used inconjunction with PMF and polarization maintaining optical components. Inanother alternative embodiment (not depicted), the polarizationcontroller 404 may be located at a different location in theinterrogator 300 than that shown in FIG. 4. For example, the laser 302may be a communication or narrow line width laser purchased inconjunction with the PMF and with the laser polarization aligned to thePMF with the polarization controller 404 located between the laser 302and the phase modulator 310.

In another alternative embodiment, the interrogator 300 may omit thepolarization splitter 402, such as when the optical fiber 112 outside ofthe interrogator 300 (including the fiber 112 comprising the FBGs 114)is PMF. In additional alternative embodiments (not depicted), theinterrogator 300 may instead comprise a polarization separatingcomponent other than the polarization splitter 402. For example, thepolarization splitter 402 may be replaced with any one or more ofpolarization filters of 0°, 45°, and 90°, and open receivers.

FIG. 5 shows an embodiment of the interrogator 300 in which the laser302, first and second optical isolators 304 a,b, optical attenuator 306,and first optical amplifier 308 of FIG. 3 are replaced with anelectroabsorption modulated laser 502 (hereinafter “EML 502”). The EML502 comprises an integrated optical isolator and an absorption regionthat acts as amplitude modulation circuitry. The controller 324 iscommunicatively coupled to the EML 502 to permit the controller 324 tocontrol amplitude modulation. Using the EML 502 instead of thecomponents in FIG. 3 that it replaces results in component and costsavings and can improve extinction performance relative to using anexternal SOA for amplitude modulation.

Referring now to FIG. 9A, there is shown an embodiment of theinterrogator 300 designed for multi-channel data acquisition in whichthere are multiple fibers 112, with each of the fibers comprisingdifferent groups of the FBGs 112 that are interrogated using TDM asdescribed above. The interrogator 300 of FIG. 9A is based on theinterrogator 300 of FIG. 5 with the addition of an optical switch 902interposed between the optical circulator 320 and the output of theinterrogator 300, and the presence of switching control circuitry 904that is communicatively coupled to and that controls operation of theoptical switch 902. The switching control circuitry 904 may be, forexample, an application specific integrated circuit, an FPGA, amicroprocessor, a microcontroller, or any other suitable type of analog,digital, or mixed signal circuitry. The control circuitry 904 may bedistinct from the controller 324 as shown in FIG. 9A or alternativelycomprise part of the controller 324 (not shown). The optical switch 902may be, for example, an EPS0116S switch from EpiPhotonics Corp. of SanJose, Calif. The switching control circuitry 904 is operable to causethe optical switch 902 to select any one of channels A, B, C, and D foroutputting the sensing and reference pulses and for receiving reflectedpulses. Channels A-D are connected to first through fourth lengths ofthe fiber 112 a-d (“first through fourth channels 112 a-d”). On each ofthe channels 112 a-d are first through third groupings of FBGs 114 d-f(“first through third FBG groups 114 d-f”). The FBGs 114 comprising thefirst FBG group 114 d are all tuned to reflect an identical, firstwavelength of light; the FBGs 114 comprising the second FBG group 114 eare all tuned to reflect an identical, second wavelength of light thatdiffers from the first wavelength; and the FBGs 114 comprising the thirdFBG group 114 f are all tuned to reflect an identical, third wavelengthof light that differs from the first and second wavelengths.

The EML 502 in FIG. 9A is configured to output light pulses at thefirst, second, and third wavelengths, thus enabling the interrogator 300of FIG. 9A to be used for wavelength division multiplexing (“WDM”). Thereceiver circuitry 322 is similarly photosensitive to the differentwavelengths of light, and consequently is able to receive and outputsignals corresponding to the interference patterns generated by thepulses sent at those different wavelengths. In alternative embodiments(not depicted), different light sources may be used; for example,several different lasers 302 may be multiplexed together and externallymodulated in a manner analogous or identical to that shown in FIGS. 3and 4 as opposed to using an absorption region as in the EML 502.

Referring now to FIG. 9B, there is shown an example of pulse timingapplicable to the interrogator 300 of FIG. 9A. In FIG. 9B, the switchingcontrol circuitry 904 instructs the optical switch 902 to transmit alongthe first channel 112 a, and the interrogator 300 then sends a firstpair of pulses 906 a along the first channel 112 a shortly after timet₀. The first pair of pulses 906 a is transmitted simultaneously usingthe first through third wavelengths corresponding to the wavelengths thefirst through third FBG groups 114 d-f are tuned to reflect,respectively. The first pair of pulses 906 a (multiplexed using threedifferent wavelengths of light) travels along the first channel 112 a,with the first pair of pulses 906 a at the first wavelength reflectingoff the first FBG group 114 d, the first pair of pulses 906 a at thesecond wavelength reflecting off the second FBG group 114 e, and thefirst pair of pulses 906 a at the third wavelength reflecting off thethird FBG group 114 f. The receiver circuitry 322 receives the threeinterference patterns between the end of the first pair of pulses 906 aand time t₁, which is shown in FIG. 9B. The receiver circuitry 322receives the interference pattern at the first wavelength as reflectedby the first FBG group 114 d, then at the second wavelength as reflectedby the second FBG group 114 e, and then at the third wavelength asreflected by the third FBG group 114 f. The switching control circuitry904 then instructs the optical switch 902 to transmit along the secondchannel 112 b, and the interrogator 300 then analogously transmits asecond pair of pulses 906 b along the second channel 112 b shortly aftertime t₁ and receives interference patterns at the three wavelengths oflight between the end of the second pair of pulses 906 b and time t₂.Similarly, the switching control circuitry 904 then instructs theoptical switch 902 to transmit along the third and fourth channels 112c,d, following which the interrogator 300 then analogously transmits athird and a fourth pair of pulses 906 c,d along the third and fourthchannels 112 c,d shortly after times t₂ and t₃ and receives interferencepatterns at the three wavelengths of light between the end of the thirdpair of pulses 906 c and time t₃ and the fourth pair of pulses 906 d andtime t₄, respectively.

In FIG. 9A, the different channels 112 a-d may correspond, for example,to different assets that the interrogator 300 is being used to monitor.For example, the different channels 112 a-d may correspond to differentpipelines that the interrogator 300 is monitoring. For any one of thechannels 112 a-d, the different FBG groups 114 d-f may correspond todifferent portions of the asset being monitored. For example, thedifferent FBG groups 114 d-f may represent different lengths of apipeline. Using multiple wavelengths to monitor different portions of asingle asset, such as a pipeline, helps to reduce reflection losses andincrease signal-to-noise ratio, since fewer of the FBGs 114 are used toreflect any one wavelength of light.

Although the interrogator 300 of FIG. 9A is based on the interrogator300 of FIG. 5, in alternative embodiments (not depicted) the opticalswitch 902 and switching control circuitry 904 may be analogously addedto any one or more of the embodiments of the interrogator 300 shown inFIGS. 3 and 4. Alternatively, the switching control circuitry 904 andoptical switch 902 may be added to other, non-depicted embodiments ofthe interrogator 300. Furthermore, although the optical switch 902 inFIG. 9A comprises the four channels 112 a-d, in alternative embodiments(not depicted), the optical switch 902 may have only two channels, onlythree channels, or more than four channels.

In another alternative embodiment (not depicted), the optical switch 902and switching control circuitry 904 may be missing, and the interrogator300 may be nonetheless be used to interrogate multiple channels. Forexample, the different channels 112 a-d can be connected in series byconnecting the end of one of the channels 112 a-d with the beginning ofanother of the channels 112 a-d. The interrogator 300 may theninterrogate the different channels using TDM. To reduce reflectionlosses, alternatively an optical circulator 320 may be placed in betweeneach pair of the channels 112 a-d, with each of the optical circulators320 redirecting reflections from the FBGs 114 directly to the receivercircuitry 322. For example, the optical circulator 320 placed betweenthe first and second channels 112 a,b redirects reflections from the FBGgroups 114 d-f in the second channel 112 b to the signal processingdevice 322.

In another alternative embodiment (not depicted), the interrogator 300may comprise the switching control circuitry 904 and the optical switch902 and be configured to transmit along multiple channels, but not use aWDM-capable light course.

In any of the embodiments herein some or all of the optical fiber 112used to connect the various optical components within the interrogator300 may be PMF and the optical components themselves may be polarizationmaintaining. As discussed above in respect of FIG. 4, maintainingpolarization between the sensing and reference pulses using PMF canincrease the interrogator's 300 sensitivity by using PMF throughout, andoptionally outside, of the interrogator 300. In variants of theembodiments of FIGS. 3 and 4, for example, PMF may be used to opticallycouple only the components between the laser 302 and the interrogator's300 output, only between the interrogator's 300 output and the receivercircuitry 322, or all optical components within the interrogator 300;and regardless of whether PMF is used to optically couple theinterrogator's 300 internal components together, PMF may be used forsome or all of the optical fiber 112 outside of the interrogator 300 andthat comprises the FBGs 114. Similarly, in the embodiment of FIG. 5, PMFmay be used to optically couple only the components between the EML 502and the interrogator's 300 output, only between the interrogator's 300output and the receiver circuitry 322, or all optical components withinthe interrogator 300; and regardless of whether PMF is used to opticallycouple the interrogator's 300 internal components together, PMF may beused for some or all of the optical fiber 112 outside of theinterrogator 300 and that comprises the FBGs 114.

In another alternative embodiment (not depicted), a high power laser canbe used as a light source in order to eliminate the output opticalamplifier 314. For example, in FIG. 5 a laser rated at at least 100 mWmay be used, and the EDFA that acts as the output optical amplifier 314may be eliminated. This helps to reduce cost and increase SNR. A highpower laser can similarly be introduced into the embodiments of FIGS. 3and 4.

In another alternative embodiment (not depicted), the controller 324 mayimplement dithering in order to reduce the effect of noise resultingfrom leakage cross-talk and spontaneous emissions, for example, andthereby increase SNR. As one example, in the embodiments of FIGS. 3 and4 the first optical amplifier 308, an SOA, is used to generate thereference and sensing pulses by modulating the amplitude of the laserlight. However, even when the amplifier 308 is off (i.e. set tocompletely extinguish the laser light) some of the laser light may stillbe transmitted through the amplifier 308; this light is referred to as“leakage”. The leakage acts as noise and impairs the interrogator's 300SNR.

The phase modulator 310 may be used to compensate for the leakage bydithering; that is, by phase modulating the leakage at a frequencysubstantially higher than the interrogator's 300 interrogationfrequency. For example, if the interrogator 300 is interrogating theFBGs 114 at a frequency of 4 MHz, the phase modulator 310 may modulatethe leakage at a frequency of 20 MHz while the amplifier 308 is off,with the phase modulation varying the phase of the leakage between 0radians and π radians. When the receiver circuitry 322 receives thereflections from the FBGs 114 the average of the leakage is zero, thusimproving the interrogator's 300 SNR relative to examples wheredithering is not used. In one embodiment, the phase modulator 310modulates the leakage at at least twice the interrogation frequency(i.e., the Nyquist frequency) or at some other even multiple of theinterrogation frequency, which provides a net DC demodulation of thedither. Modulating the leakage at at least 2.5 times the interrogationfrequency provides a potentially useful buffer between the modulationfrequency and the Nyquist frequency. Modulating at higher noise ditherrates, such as at at least ten times the interrogation frequency, insome embodiments permits analog filtering to be applied to the signalthe interrogator 300 receives from the FBGs 114 to reduce costs. Forexample, in one embodiment, modulating the leakage at a rate of at leastone hundred times the interrogation frequency prevents the leakage frombeing able to pass the bandwidth of the receiver circuitry 322, thuspermitting noise filtering without having to add specialized filteringcircuitry over and above what is depicted in FIGS. 3-5.

Calibration

Referring now to FIG. 8, there is shown a method 800 for calibrating theinterrogator 300, according to another embodiment. The method 800 may beencoded on to the FPGA that comprises the controller 324 as acombination of FPGA elements such as logic blocks. The method 800 isdescribed below in conjunction with the interrogator 300 of FIG. 5,although it may also be performed using other embodiments of theinterrogator 300, such as the embodiments of FIGS. 3 and 4.

When performing the method 800, the controller 324 begins at block 802and proceeds to block 804 where it transmits a calibration pulse to theFBGs 114. This calibration pulse may or may not be phase delayed usingthe phase modulator 310. The calibration pulse is reflected off each ofthe FBGs 114 and the reflected pulses return to the interrogator 300 andare received by the receiver circuitry 322 (block 806). The pulse thatreflects off the first FBG 114 a returns to the receiver circuitry 322first and has the highest amplitude of the reflected pulses; the pulsethat reflects off the second FBG 114 b is the second reflected pulse toarrive at the receiver circuitry 322 and has the second highestamplitude, and this pattern continues for the reflections off theremaining FBGs 114. The controller 324 at block 808 determines thetiming between the sensing and reference pulses based on differences inwhen the reflections of the calibration pulse are received at thereceiver circuitry 322. In one embodiment, second order reflections fromthe FBGs 114 (i.e., reflections of reflections) are mitigated usingdigital signal processing techniques such as infinite impulse responseor finite impulse response filters, or through suitable modulation ofthe sensing and reference pulses such as with Barker codes.

If, for example, the FBGs 114 are equally spaced along the optical fiber112 then intervals between any two consecutive reflections haveidentical durations. The controller 324 can store this duration andcontrol pulse generation such that the interval between the sensing andreference pulses is of this duration. As another example, if the FBGs114 are not equally spaced along the optical fiber 112 then the intervalbetween receipt of the calibration pulse reflections from any two of theFBGs 114 is the interval between the sensing and reference pulses thatshould be used if interferometry is to occur as a result of reflectionsoff those two FBGs 114. For example, if the interval between thecalibration pulse reflections from the first and second FBGs 114 a,b ist₁ and the interval between the calibration pulse reflections from thesecond FBG 114 b and a third FBG 114 is t₂, with t₁≠t₂, then thecontroller 324 generates the sensing and reference pulses with a delayof t₁ between them if interference is desired between reflections fromthe first and second FBGs 114 a,b and with a delay of t₂ between them ifinterference is desired between reflections from the second FBG 114 band the third FBG 114.

In addition to timing between the reference and sensing pulses,calibration pulses can be used to level power between multiple laserswhen wavelength division multiplexing is being used, adjust gain of thevarious amplifiers 308,314 in the interrogator 300, and determinespacing between the FBGs 114.

Calibration using the calibration pulse can be done at initial setup ofthe interrogator 300 or periodically while using the interrogator 300 tointerrogate the optical fiber 112. The interrogator 300 can berecalibrated as desired; for example, depending on factors such asthermal changes, mechanical changes (e.g. geotechnical shifts), and longterm fiber stretching, the interrogator 300 can be recalibrated everyfew seconds, minutes, hours, or longer. As calibration is done inreal-time, any data related to the phase of the reflected pulses that ismissed as a result of being received during calibration can beapproximated using interpolation. Interpolation can be performed usingan intelligent reconstruction filter such as a linear or cubicinterpolator.

As discussed above, the interrogator 300 may comprise a single laserthat is used as a light source within the interrogator 300, andalternatively (as depicted in FIG. 9A, for example) the interrogator 300may comprise multiple light sources of different wavelengths multiplexedtogether to enable WDM.

Furthermore, while the phase modulator 310 in the above embodiments is alithium niobate phase modulator, in alternative embodiments (notdepicted) different types of phase modulators may be used. Examplealternative phase modulators are gallium arsenide phase modulators andindium phosphide phase modulators. The phase modulator 310 may or maynot be a Mach Zehnder-type modulator.

Aside from an FPGA, the controller 324 used in the foregoing embodimentsmay be, for example, a processor, a microprocessor, microcontroller,programmable logic controller, or an application-specific integratedcircuit. For example, in one alternative embodiment, the controller 324collectively comprises a processor communicatively coupled to anon-transitory computer readable medium that has encoded on it programcode to cause the processor to perform one or both of the examplemethods of FIGS. 7 and 8. Examples of computer readable media arenon-transitory and include disc-based media such as CD-ROMs and DVDs,magnetic media such as hard drives and other forms of magnetic diskstorage, semiconductor based media such as flash media, random accessmemory, and read only memory.

It is contemplated that any part of any aspect or embodiment discussedin this specification can be implemented or combined with any part ofany other aspect or embodiment discussed in this specification.

For the sake of convenience, the example embodiments above are describedas various interconnected functional blocks. This is not necessary,however, and there may be cases where these functional blocks areequivalently aggregated into a single logic device, program or operationwith unclear boundaries. In any event, the functional blocks can beimplemented by themselves, or in combination with other pieces ofhardware or software.

While particular embodiments have been described in the foregoing, it isto be understood that other embodiments are possible and are intended tobe included herein. It will be clear to any person skilled in the artthat modifications of and adjustments to the foregoing embodiments, notshown, are possible.

We claim:
 1. A method for interrogating optical fiber comprising fiberBragg gratings (“FBGs”), the method comprising: (a) generating a firstpair of light pulses and a second pair of light pulses from phasecoherent light emitted from a light source, wherein the light pulses aregenerated by modulating the intensity of the light without splitting thelight, and wherein the second pair of light pulses immediately followsthe first pair of light pulses; (b) for each of the first and secondpairs of light pulses, applying a phase shift to at least one of thelight pulses in the pair relative to the other of the light pulses inthe pair, wherein the phase shift applied to the at least one of thelight pulses in the first pair of light pulses and the phase shiftapplied to the at least one of the light pulses in the second pair oflight pulses are the same; (c) transmitting the light pulses along theoptical fiber; (d) receiving reflections of the pulses off the FBGs; and(e) determining whether an optical path length between the FBGs haschanged from an interference pattern resulting from the reflections ofthe pulses.
 2. The method of claim 1 wherein determining whether theoptical path length has changed comprises converting the interferencepattern from an optical to an electrical signal.
 3. The method of claim1 wherein a phase modulator is used to phase shift the at least one ofthe light pulses, the phase modulator selected from the group consistingof a lithium niobate phase modulator, a gallium arsenide phasemodulator, and an indium phosphide phase modulator.
 4. The method ofclaim 1 wherein polarization of the light pulses is maintained from whenthe light pulses are generated until the light pulses are transmittedalong the optical fiber.
 5. The method of claim 4 further comprisingsplitting the polarization of the reflected pulses prior to convertingthe interference patterns.
 6. The method of claim 1 wherein polarizationof the light pulses is maintained from when the light pulses aregenerated until the interference pattern resulting from the reflectionsof the pulses is observed.
 7. The method of claim 1 wherein the lightsource is a laser and the intensity of the light is modulated using afirst optical amplifier external of and optically coupled to the laser.8. The method of claim 1 wherein the light is generated by anelectroabsorption modulated laser and the intensity of the light ismodulated using an absorption region comprising part of the laser. 9.The method of claim 1 wherein the light source comprises a laser havinga power of at least 100 mW.
 10. The method of claim 1 wherein theapplying the phase shift comprises applying a positive phase shift to afirst pulse and applying a negative phase shift to a subsequent, secondpulse intended to interfere with the first pulse.
 11. The method ofclaim 10 wherein the first and second pulses differ in phase from eachother by more than π radians.
 12. The method of claim 10, wherein amagnitude of the positive phase shift is equal to a magnitude of thenegative phase shift.
 13. The method of claim 1 further comprising: (a)transmitting a calibration pulse to the FBGs; (b) receiving reflectionsof the calibration pulse off the FBGs; and (c) based on differences inwhen the reflections of the calibration pulse are received, determiningtiming between the sensing and reference pulses.
 14. The method of claim1 wherein applying the phase shift comprises applying a non-linear phaseshift or a piecewise linear phase shift to the at least one of thepulses.
 15. The method of claim 14 wherein the phase shift is a Barkercode.
 16. The method of claim 1 further comprising dithering leakagefrom the light source by phase shifting the leakage between 0 and itradians at a frequency at least 2.5 times higher than a frequency atwhich interrogation is being performed.
 17. An optical fiberinterrogator for interrogating at least two optical fibers comprisingfiber Bragg gratings (“FBGs”), the interrogator comprising: (a) a lightsource operable to emit phase coherent light; (b) amplitude modulationcircuitry optically coupled to the light source and operable to generatepulses from the light, wherein the pulses are generated withoutsplitting the light; (c) an optical switch optically coupled to thelight source and comprising at least two output channels including afirst output channel and a second output channel, the optical switchoperable to switch transmission of light between each of the at leasttwo output channels; (d) control circuitry, communicatively coupled tothe amplitude modulation circuitry and to the optical switch, configuredto perform a method for interrogating each of the at least two opticalfibers comprising generating first and second pairs of light pulses forthe first output channel, and third and fourth pairs of light pulses forthe second output channel, by using the amplitude modulation circuitryto modulate light output by the light source and interrogating each ofthe at least two optical fibers by using the optical switch to switchtransmission amongst the at least two output channels, wherein thesecond pair of light pulses immediately follows the first pair of lightpulses, and wherein the fourth pair of light pulses immediately followsthe third pair of light pulses; and (e) a phase modulator configured toapply, for each of the first, second, third, and fourth pairs of lightpulses, a phase shift to at least one of the light pulses in the pairrelative to the other of the light pulses in the pair, wherein one ormore of: (i) the phase shift applied to the at least one of the lightpulses in the first pair of light pulses and the phase shift applied tothe at least one of the light pulses in the second pair of light pulsesare the same; and (ii) the phase shift applied to the at least one ofthe light pulses in the third pair of light pulses and the phase shiftapplied to the at least one of the light pulses in the fourth pair oflight pulses are the same.
 18. The interrogator of claim 17 wherein thelight source is operable to emit multiple wavelengths of light forinterrogating different groups of the FBGs using wavelength divisionmultiplexing.
 19. A method for interrogating at least two optical fiberscomprising fiber Bragg gratings (“FBGs”), the method comprising: (a)generating first, second, third, and fourth pairs of light pulses fromphase coherent light emitted from a light source, wherein the first,second, third, and fourth pairs of light pulses are generated bymodulating the intensity of the light without splitting the light,wherein the second pair of light pulses immediately follows the firstpair of light pulses, and wherein the fourth pair of light pulsesimmediately follows the third pair of light pulses; (b) for each of thefirst, second, third, and fourth pairs of light pulses, applying a phaseshift to at least one of the light pulses in the pair relative to theother of the light pulses in the pair, wherein one or more of: (i) thephase shift applied to the at least one of the light pulses in the firstpair of light pulses and the phase shift applied to the at least one ofthe light pulses in the second pair of light pulses are the same; and(ii) the phase shift applied to the at least one of the light pulses inthe third pair of light pulses and the phase shift applied to the atleast one of the light pulses in the fourth pair of light pulses are thesame; (c) transmitting the first and second pairs of light pulses alongone of the optical fibers (“first optical fiber”); (d) transmitting thethird and fourth pairs of light pulses along another of the opticalfibers (“second optical fiber”); (e) receiving reflections of the firstand second pairs of light pulses off the FBGs along the first opticalfiber; (f) receiving reflections of the third and fourth pairs of lightpulses off the FBGs along the second optical fiber; and (g) determiningwhether an optical path length between the FBGs on the first and secondoptical fibers has changed from an interference pattern resulting fromthe reflections of the first second, third, and fourth pairs of pulses.20. The method of claim 19 wherein at least one of the first and secondoptical fibers comprises different groups of the FBGs tuned to reflectlight of different wavelengths, and wherein one or more of: (a) one ormore of the first and second pair of light pulses are transmitted usingwavelength division multiplexing using the wavelengths that the FBGs aretuned to reflect; and (b) one or more of the third and fourth pairs oflight pulses are transmitted using wavelength division multiplexingusing the wavelengths that the FBGs are tuned to reflect.